Eye and eye lifting slings exist in various forms made of metals and synthetics in single element form and in multi part or element form. In metal or wire rope a sling may be formed by utilizing a single length of wire and forming an eye in each end by splicing, swaging, or potting. In synthetic form a sling may be formed similarly by utilizing a single length of rope (of any construction such as 3 strand, single braid, double braid, parallel, plaited, etc.) and forming an eye in each end by splicing, swaging, knotting, potting, etc. Flat synthetic webbing is also widely used to make slings by folding an eye in each end and stitching the bitter end to the standing part of the webbing, thus forming eyes that can be attached between an object to be lifted and to an apparatus designed to exert a lifting force. Synthetic slings are also formed by utilizing a strength element such as a twisted strand of fibers (or braided element) and laying a continuous length in a circular path making multiple laps until a desired combined strength is achieved and then enclosing these strands within a “sock” of suitable cloth type material.
Each of these various types of slings has advantages and disadvantages. The biggest difference between wire based slings versus synthetic slings is weight. For a given lift capacity, the synthetic alternative is 4 to 10 times lighter. Wires' principle advantages are high abrasion resistance, high UV resistance, high temperature tolerance, and cheaper initial cost. Its disadvantages are high weight, stiffness, low corrosion resistance, abrasive to other objects, high conductivity, loss of strength in smaller bend diameters, difficulty of inspection (because of weight) and high recoil and spring-back. Synthetic slings (of high strength fibers such as aramids, ultra high molecular weight polyethylene, liquid crystal polymers, etc.) are much lighter to handle, non-corrosive, non-abrasive to other objects, very flexible, easy to store and have better strength retention over small diameter pins and lift hooks, and have low to no conductivity.
The disadvantages of current synthetic slings are higher cost, lower tolerance to high temperatures, difficult to inspect (sleeve enclosed strength fibers), cannot be pushed (as in under objects), lower tolerance to UV degradation, prone to contamination and moisture penetrating to the strength elements, easily cut and bulky. When wire slings are fabricated for higher lifting capacity, a typical method is to use multiple strands or “parts” of a given size of wire. This is primarily done because of the difficulty of bending a larger single wire rope into a manageable eye size and the associated loss of strength when bent too sharply. Typically, the wire is fabricated into a 3 part (or pass) configuration. Then two or three “matched” sets of the 3 part slings are combined to form a 6 or 9 part sling. The current invention is an improvement over this type of multipart sling utilizing synthetic strength elements configured or fabricated in a more efficient product, such that the advantages of wire style and synthetic style slings are embodied while eliminating or minimizing the disadvantages.
What is needed, therefore, are techniques for manufacturing synthetic slings of lower cost and higher performance.